Method of anchoring coatings



' plied to sheet 7 Patented 7, 1941 2,258,434 METHOD or ancnonmo cosmos Allen Abrams, George W.

and Winfred H.

Brabender, Wausau,

Forcey, and George J.

Graebner,

Neenah, Wis., assignors to Marathonv Paper Mills Company, Rothschild, Wis.,

of Wisconsin a corporation No Drawing. Continuation or application Serial. No. 269,781, April 24, 1939. This application- February 9, 1940, Serial No. 318,180

This application is a continuation of application Serial No. 269,781, filed April 24, 1939.

This invention relates to a method for anchoring coatings to fibrous sheet material having a closely formed surface, such as vegetable parchment and similar sheet materials having a relatively high strength when wet.

Recently there have been developed many types of thermoplastic coating materials of varying compositions and for various applications. These coatings may possess one or more of the follow ing properties: waterproofness, moistureproofness, flexibility,

melt compositions or dissolved in volatile solvents, then applied-with subsequent evaporation of the solvent. I The coated sheets may be used for a variety of purposes, particularly for packaging purposes in the food industry. One use of considerable importance requires a coated sheet which has a waterand moistureproof, flexible, heatsealing coating. This sheet is employed for makingbags, or as a wrapper, for the packaging of frozen foods. The foods to be frozen include dilferent kinds of berries, fruits, meats, and other commodities. In many cases they have a very high water content when placed in the package. Moreover the froz; en containers, during subsequent thawing may condense water on the outside surface. Hence it is necessary that the packaging material resist water; and that both the coating and the base sheet material should'not disintegrate when wet.

Papers of high wet strengt are usually em.- ployed for such packages. These papers may be of the vegetable parchment type or they may heat sealing. They may be ap v materials in the form of "hot 1 Claims. (.01. iii-ea) Patent 2,054,113, September 15, 1936. The coat.- ing composition comprises a waxy thermoplastic constituent and a compatible film-forming constituent. The coating composition may, for example, consist of a thermoplastic composition comprising pale crepe rubber and paraflln-wax, the rubber content ranging preferably from about 6 to 50 by weight of the composition. The composition is compounded under controlled conclitons so as'to have a viscosity not less than 8000 sees. (50 0. c. measured on a Scott vlscosimeter at 90 C.) and is applied in hot condition to the base sheet by a hot roll or'doctor bar. Instead of paraffin-wax other natural or synthetic waxy substances may be used such as beeswax, amorphous waxes, candelilla, spermaceti, carhauba, hydrogenated oils such as cottonseed and fish oils. and

derivewet strength from treatmentssuch as with viscose, glue-formaldehyde or with certain resins-.- for example, of the urea formaldehyde type.

.When certain coatings are applied to a dense, closely-formed sheet such as vegetable parchment, the anchorage is largely of a superficial type. -Whlle the sheet is dry, good adhesion of the coating material may be obtained;- but when the sheet is soaked for sometime in water the coating may loosen and separate from the paper.

This is particularly true if the base paper has been softened with awater-miscible, hygroscopic substance such as glycerine or invert sugar. Since the coating is generally used not only for waterproofing but also for heat sealing the fllled'bag. any separation between the coating and the paper may be the cause of considerable annoyance.

A coated paper of the type frequently used for frozen food packages is described in United States the like, singly or mixtures thereof, all oi!v which substances are included in the term wax as used in the appended claims.

'The' film-forming constituent may be india rubber, balata, gutta percha, smoked sheet rubber, chlorinated rubber, rubber hydrochloride, isoprene polymers, Vistanex (isobutylene polymers) and the like, all of which substances are intended to be equivalents for the term rubber as used in the appended claims. The coating composition may be applied to the base sheet in hot molten condition or in a suitable solvent such as benzol, zylol, carbon tetrachloride, acetone, alcohols, ethers or esters and the like. 'When such a coated sheet is made with a base of wet strength paper it finds wide acceptance because of its unique properties.

the sheet is soaked in water, the coating often separates from the base wet strength" paper, particularly ii. a closely formed sheet such as parchment is used.

Our invention therefore relates to a method of securing tight adhesion of the coating to the fibrous base sheet, especially such as vegetable parchment and the like, when the coated sheet solids. A typical analysis of the thick size from which this emulsion is prepared may be as follows: solids water 30%. Analysis of the solids shows free rosin 33 combined rosin 56%, soda ash 11% We prefer to use low concentra- However, one disadvantage has been that, if

tion of size since higher concentrations (e. g.

' 25%) do not work satisfactorily.

After the sheet has passed through the size bath, any excess liquid is removed by running it through squeeze rolls. Then the sheet is again passed into a bath containing an acid coagulating or setting solution such as ordinary paper makers alum (chemicallyknown as aluminum sulphate).

. thermoplastic coating consisting, for example, of

rubber and wax which will. not separate when the sheet is wet. The addition of say ,8-% of a resin such as ester gum, glyptal resins, phenol resins, rosin and the like improves the adhesion.

We are fully aware of the fact that a treatment of rosin size, followed by alum, is customarily employed in the paper industry for obtaining water resistance in pulp and paper products, but it is not used on vegetable parchment and similar sheet material. However, our treatment is for an entirely different purpose and accomplishes a different result. As previously indicated, our treatment permits of anchoring thermoplastic coatings to surfaces which are so closely knit that it is not possible to get actual penetration of the coating and bonding into the surface of the sheet. Without such bonding or anchorage, wetting of the sheet will cause the coating to loosen and separate; but with our treatment the coating remains attached to the sheet.

It is important to note that, for proper adhesion of the coating which will not separate when the sheet is wet, a number of conditions must be fulfilled, since the bonding material must (1) be impervious to water (2) anchor suitably to the sheet material (3) be miscible or compatible with the coating material. Thus, other resinous or equivalent materials may be used'provided they fulfill the foregoing requirements.

Whereas softening of vegetable parchment, and the like, sheets may be accomplished on the paper machine or by subsequent operation, we find that this may be included with our treatment. Forv example, the glycerine, invert sugar, etc. may be combined with the rosin size emulsion and with the alum solution, so that the softening is carried out simultaneously with the treatment for anchoring coating. I

It is to be understood that numerous changes and modifications may be-made in our procedure and in the compositions used which are intended to be included within the scope of the appended claims.

We claim: 1. A coated sheet material comprising a fibrous base sheet having a closely formed surface, a bonding layer on said surface containing rosin size and a thermoplastic coating layer comprising essentially wax and rubber united to said bonding layer, said layers remaining attached when the said sheet is wet.

2. A coated sheet material comprising a fibrous base sheet havinga closely formed surface, a precipitated bonding layer of rosin and aluminum resinate on said surface and a thermoplastic coating layer comprising paraffin-wax and 6 to 50% by weight of rubber united to said bonding layer, said layers remaining attached when the said sheet is wet.

3. A coated sheet material comprising a base sheet of vegetable parchment having a closely formed surface, a bonding layer on said surface containing rosin size and a thermoplastic coating layer comprising parafiin-wax and 6 to 50% by weight of rubber united to said bonding layer,

said layers remaining attached when the said sheet is wet.

4. A coated sheet material comprising a base sheet of vegetable parchment having a closely formed surface, a bonding layer containing rosin size on said surface and a thermoplastic coating layer comprising paraflin-wax and 6 to 50% by weight of pale crepe rubber having, when applied, an initial viscosity of 8,000 seconds '(50 cc. measured at C. on a Scott viscosimeter), said layers remaining attached when the said sheet is wet. A

5. The method of anchoring thermoplastic coatings upon a sheet of vegetable parchment which comprises coating said sheet material with a. bonding material comprising essentially rosin size, and anchoring to said bonding material a thermoplastic coating composition comprising essentially wax and rubber miscible with said bonding material and compatible therewith.

6. The method of anchoring thermoplastic,

coatings upon a sheet of vegetable parchment which comprises passing the sheet through a bath of rosin size emulsion, passing the sheet through a coagulating bath, drying such sheet and then applying thereto a thermoplastic coating composition comprising essentially wax and rubber.

7. The method of anchoring thermoplastic coatings upon a fibrous sheet having a closelyformed surface which comprises coating said sheet with a bonding material comprising 25-- sentially rosin size, and anchoring to said bonding material a thermoplastic coating composition comprising essentially wax and rubber.

ALLEN ABRAMS.

GEORGE w. FORCEY. GEORGE J. BRABENDER. WINFRED H. GRAEBNER. 

